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Saturday, 8 July 2017

Establishing the truth of Arctic sea ice melting

Arctic
sea ice is ebbing faster than normal, and by September it could
bottom out at a very low level

By
Tom Yulsman

This
animation consists of false-color images of the Russian coast
and adjoining East Siberian Sea acquired by NASA’s Aqua satellite.
On June 18, the offshore waters were choked with sea ice. By
July 6, 2017, a lot of it had broken up. In the false-color scheme,
land is green, black is indicative of open water, and ice is a
light turquoise. The darker blue prominent in the June 18th
image probably is indicative of melting snow and ice
that’s causing liquid water to accumulate. (Images: NASA
Worldview. Animation: Tom Yulsman)

Under
frigid winter conditions, the Arctic’s floating lid of sea ice
typically expands to a maximum extent in March. But thanks
to human-caused global warming, that maximum seasonal spread of
the ice has been shrinking over the years — and this past
March it reached the lowest
level ever
observed for the month.

But
then, something unexpected happened: In May, sea ice retreated much
more sluggishly.

This
prompted some people who deny the reality of humankind’s impact on
the climate to pounce. “ARCTIC SEA ICE BOUNCES
BACK,” shouted one
headline (on
a blog that bills itself as being dedicated to “common sense
on climate change”). Well, hold on…

Now
we have the monthly report from
the scientists at the National Snow and Ice Data Center, and wouldn’t
you know it: During June, Arctic sea ice shrank faster than average.
That resulted in June’s ice cover coming in at sixth lowest in
the satellite monitoring era, which began in 1979.

But
that’s not the most newsworthy part of the report. In my view, this
is: By July 2, Arctic sea ice had retreated to a
particularly low level matching what was seen on the same date
in 2012.

That’s
significant because in 2012, sea ice went on to set a record
that has not been broken since. During the summer of that year,
ice extent dropped precipitously, and in September it went lower
than ever seen before.

A
comparison of true-color and false-color images acquired by the Aqua
satellite on June 18, 2017 in the same area as shown in the
animation at the top of this post. Note the likely melt ponds, which
contribute to the break up of sea ice. (Images: NASA Worldview.
Animation: Tom Yulsman)

Of
course, just because Arctic sea ice melted relatively fast in June
and is really low now doesn’t mean it will set a new record this
coming September. But given the conditions in the Arctic
right now, the odds are good that the sea ice extent minimum
will be very low.

How
low? It’s impossible to know for sure at this point. But scientists
of the Sea Ice Prediction Network do their best to make
informal forecasts. For their June
report,
they analyzed predictions from 33 different sources.
According to the report, the median forecast is for Arctic sea ice
extent to bottom out in September at 4.43 million square
kilometers. That would put it in the top five of lowest extents on
record.

That
relatively low number probably reflects a number of factors. One is
temperature — specifically, how cold has it been in the Arctic and
for how long?

Source:
NSIDC

This
is gauged by something called cumulative freezing degree days. This
is the sum of daily mean temperatures below freezing over a
particular period.

Based
on this metric, between July of last summer and July of this year,
the Arctic has been “extremely mild,” according to the NSIDC
report. Do keep in mind that this is a relative statement. It
is a specific comparison to a base period of 1981 to 2000.

You
can see just how relatively mild it has been by clicking on the
graphic above. From July of 2016 to now, the period has been milder
than the same period in 2012, the year Arctic sea ice
hit its lowest extent on record.

The
image above shows what’s happening. Areas in blue
indicate areas where Arctic sea ice is thinner than the 2000 to
2015 average. And as the graphic shows, sea ice thicknesses for
May 2017 were below average over most of the Arctic Ocean. (The
exception: the red areas north and west of the Svalbard archipelago.)

The scrawny
ice cover will make it easier for warm summer temperatures to
melt the ice. As the June NSIDC report states, “Starting the melt
season with below-average ice thickness raises the likelihood of
having especially low September ice extent.”Time will tell.